It is well known that, in providing compressed gas for industrial processes and other applications, it is usually necessary to eliminate moisture from the compressed gas system in order to prevent corrosion and contamination, which eventually impair the proper functioning of components within the system. In the past, various arrangements have been, employed which utilized cooling devices, desiccants and other instruments for dehydrating and purifying moisture laden compressed gas. As a general rule, it is desirable to use drying equipment capable of producing consistently low dewpoints regardless of ambient temperature or seasonal fluctuations.
The general manufacturing industry today is a predominant user of refrigeration gas drying equipment for removing water from compressed gas. In such apparatus, the moist gas to be dried is flowed through a heat exchange chamber which cools the point where moisture condenses and is drained from the gas. Refrigeration-type dryers have remained popular because of their economical cost, easy maintenance, simple installation requirements and low energy consumption. However, experience has demonstrated that refrigeration dryers are somewhat restrictive in certain applications because they generally produce dewpoints in the range of 35.degree. F.-50.degree. F. and also exhibit some freezing problems in low ambient temperatures.
Another type of prior art gas drying system which has proved highly satisfactory for use in industrial processes is the desiccant or adsorption dryer which dries gas by adsorbing water molecules onto the surface of an adsorbent drying agent or desiccant such as alumina or silica gel. In systems of this type, the gas to be treated is transmitted through a desiccant bed until it becomes substantially saturated with moisture or other impurities. At this point, removal of moisture by the desiccant bed is terminated, and the bed is reactivated or regenerated by heating the bed such that the absorbed moisture and impurities are evaporated and transferred away. In continuous production of dry gas, a pair of adsorbent beds is employed so that one of the beds is drying gas while the other is being regenerated. One such system is disclosed in U.S. Pat. No. 3,766,660 issued Oct. 23, 1973, to the inventor of the current invention; see also U.S. Pat. No. 4,783,432.
Desiccant dryers are particularly desirable in some applications because they are capable of producing -40.degree. F. dewpoints. However, their performance is dependent upon desiccant regeneration, and the temperature and humidity of the gas used in the regeneration greatly affects dewpoint production. Although most compressors produce sufficient heat of 300.degree. F.-50.degree. F. for regeneration, it has been found that, in some types of desiccant dryers of the heated regenerated type, extraneous energy sources, such as booster heaters, are necessary to increase regeneration quality. Unfortunately, such additional requirements increase the cost in equipment and energy expended, and generally create further maintenance problems. The art has previously attempted to address these problems by providing adsorption dryers which utilize the heat of compression as regeneration energy for the adsorbent beds. In other words, there have been arrangements designed which extract the heat energy from the compressed gas for application in reactivating the saturated adsorbent bed.
One such interesting design is shown in U.S. Pat. No. 3,568,406 issued Mar. 9, 1971 to Dynes. In this desiccant dryer, most of the gas to be dehydrated is fed through a cooler and a first adsorbing desiccant, while a portion of the very gas to be dehydrated is employed to reactivate a second saturated desiccant which is used for extracting the aqueous constituents from the system. Although such arrangements have been an advance in the right direction, the energy efficient results have not been universally applicable to all compressors. Especially problematical are oil flooded screw compressors which do not have discharge temperatures high enough for regeneration and are saturated with cooling media vapor. Utilization of the Dynes '406 patent with an oil flooded screw compressor would allow hot, wet oily gas to bypass the cooler and be introduced to the adsorbent beds, fouling or clogging the desiccant and thereby shortening its life and the efficiency of the entire system.
For many of the same reasons explaining the popularity of refrigeration dryers, oil flooded screw compressors are in prevalent use in manufacturing environments throughout the world. However, to date the art has not responded adequately to the needs of the manufacturing industry for a dryer producing clean, dry gas at a dewpoint of -40.degree. F. which can be used efficiently in conjunction with an oil flooded screw compressor. It is particularly desirable that such a drying scheme address the feasibility of utilizing the relatively low 170.degree. F.-220.degree. F. discharge temperature of the heated oil from the screw compressor, which heretofore has been wasted, to effect regeneration of the dryer.
At best, the prior art has contemplated the utilization of heated oil to simply heat compressed air. Such teaching is found in U.S. Pat. No. 3,785,755 issued Jan. 15, 1974 to Novak et al. While this system does reheat and lower the relative humidity of the air, it does not reduce the dewpoint lower than that leaving the moisture separator of the system and, notably, fails to include any dryer. This system continually reheats the compressed air with heated oil and is dependent upon this continual heat for function.
From the foregoing, it can be seen that various attempts have been made by the prior art to treat compressed air. However, there remains a need in this well-developed art for a compressed gas system which furnishes a low dewpoint gas product by applying the compressor oil heat as regeneration energy to desorb moisture from, the, adsorbent bed and which effectively marries a desiccant dryer with an oil flooded screw compressor.